1. Field of the Invention
The invention relates to a method for controlling the thickness of a substrate, for example a glass or plastic substrate, and also relates to a substrate produced in this way.
2. Description of Related Art
Methods for producing substrates, for example glass or plastic substrates, have long been known. For example, document DE 101 28 636 C1 discloses a method in which the thickness of a flat glass is selectively influenced, and also a device for carrying out such method. In this case, a flat glass is conveyed, directly after shaping, past a device which extends over the entire width of the glass ribbon and which causes controlled cooling of the glass, and moreover the glass can be heated selectively and adjustably over its entire width, through local heat supply by means of a laser beam. This laser beam is driven across the width of the glass ribbon at a high frequency, and its output power is selectively adjusted, so that an appropriate heating power is achieved with spatial resolution. In this manner, a flat glass is obtained which has a most constant thickness possible across its width.
Furthermore, DE 10 2008 063 554 A1 describes a method and an apparatus for producing flat glass, in which the thickness can also be selectively influenced over the width of the glass. In this case, a glass ribbon is drawn through a slotted nozzle and is then guided into a drawing chamber, which is configured so as to have one wall exhibiting locally varying radiation absorption and/or thermal conductivity at least in sections across the width of the glass ribbon. Furthermore, for promoting thickness control, a laser beam may locally influence the glass ribbon. Additionally, a gas flow may also selectively locally influence the thickness of the ribbon. In this way, a glass ribbon is obtained with a desired thickness profile adjustable across the width thereof, for example with a greater glass thickness in the center of the ribbon than towards the edges thereof.
U.S. Pat. No. 8,904,822 B2 discloses a method by which a glass or plastic substrate with controlled thickness is obtained. In this method, a glass or plastic ribbon is drawn by applying a tension at the edges of the ribbon. Furthermore, the thickness of the ribbon is determined and controlled. If a deviation in thickness is detected, the portion of this thickness deviation is selected, this portion being in a viscous state. Then, this selected portion which is in a viscous state is heated by directing a laser beam thereto. Due to the heating, the portion then attains the predetermined thickness. The heating comprises controlling the laser power, the holding time of the laser on the selected portion, and/or adjustment of the wavelength of the laser.
However, all of these methods have a number of drawbacks.
For example, in all cases the material is drawn by exerting a pulling force, for example by so-called edge rolls contacting the glass in the edge portions of the ribbon. However, this causes surface damage of the ribbon at locations contacted by the edge rolls, so that in this way it is not possible to utilize the entire width of the ribbon.
If, for example, a gas flow acts on the ribbon via nozzles for controlling the thickness, waviness will additionally be caused across the net width of the ribbon as a result of the width of the nozzle and the spacing between the surface cooled by the nozzle and the glass. For example, the resolution of such nozzles is approximately 30 mm, so that in this way a slight waviness with a periodicity or wavelength of less than 30 mm cannot be eliminated in the ribbon. Here, net width refers to the portion of the glass ribbon in which the properties of the glass ribbon are within the predetermined specifications. The net width of a glass ribbon is therefore the width of the quality portion of the glass ribbon and usually results from the drawn glass ribbon when separating the edge portions also referred to as borders.
Furthermore, measures for influencing the thickness of a glass or plastic ribbon are taking effect in a region in which the ribbon is in the viscous state. This viscous state is defined as a region in which viscosity is greater than 105 dPa·s, as can be seen, for example, from U.S. Pat. No. 8,904,822 B2. In order to guarantee sufficient heating of the material so that the material of the ribbon is sufficiently flowable in order to achieve a reduction in thickness, very high output powers have to be applied in this manner. However, the process not only becomes cost-intensive in this way, but additionally the high energies or output powers of the laser beam impede thickness control to be performed with sufficient precision, so that some variations in thickness across the net width of the glass or plastic ribbon continue to exist.
Thus, there is a need for a method for controlled adjustment of the thickness of a glass or plastic ribbon, which mitigates the existing drawbacks of the prior art.